1. Field of the Invention
The invention generally relates to gamma ray detectors. More particularly, the invention relates to an improved gamma ray detector that can more accurately determine the position of gamma ray interactions within the detector for producing an image of a scanned object.
2. Background
Gamma ray detectors are used in a wide variety of devices and processes including, for example, positron emission tomography (PET), single photon emission-computed tomography (SPECT), contraband explosive detectors, and others. All such devices incorporate detectors capable of determining, with some degree of accuracy, the position of interactions between gamma rays and the detector components. By accumulating position measurement data corresponding to a large number of such interactions, an image of a scanned object from which the gamma rays are being emitted can be produced. These techniques are well known to those of skill in the art and need not be detailed herein.
A difficulty encountered with these detectors is that in order to scan an object efficiently and with sufficient resolution, a multiplicity of such detectors are necessary, and the positions of interactions of gamma rays with the detectors must be determined so that with a plurality of such determinations (typically in the millions) sufficient data is obtained to produce an accurate image of the object being scanned. Because each detector must be capable of generating position data for gamma ray interactions that occur within that detector, the equipment conventionally used in the acquisition of the position data, as well as the subsequent compiling and image generation (e.g., by a computer) tends to require a relatively large and expensive apparatus.
Typically, the collection of detection data by such a detector is initiated by the interaction of a gamma ray with a scintillator material provided within the detector that generates light. By determining in which of the plurality of detectors the light was emitted and the position within the triggered detector from which the light was emitted, a data point presumptively corresponding to a positron annihilation event within the scanned object is collected. By arranging a multiplicity of such detectors around at least a section of the scanned object, a large number of data points can be collected and then subsequently processed by a computer to generate an image corresponding to the collected detector data.
Conventional gamma ray detectors used in such gamma ray scanning devices typically include an expensive scintillating crystalline material (e.g., cerium doped lutetium oxyorthosilicate (LSO) and/or bismuth germinate (BGO)) that will emit light when triggered by a gamma ray. The X-Y position resolution of such detectors is typically on the order of 20 mm2 and tends to exhibit some degree of non-uniformity in this resolution across the detector array. As a result, such detectors have an inherent level of inaccuracy with regard to the precise position (i.e., in X-Y coordinates) at which the interaction occurred. In addition, the depth of the interaction (i.e., the Z coordinate) is generally not determined, or is poorly determined, resulting in a so-called parallax error and further reducing the accuracy of the image generated from the position data.
A modular light signal triggerable detector is disclosed in Bryman's U.S. Pat. No. 6,100,532, entitled “Detector for Gamma Rays” (“Bryman I”) which is hereby incorporated by reference in its entirety. Bryman I discloses a gamma ray detector for determining the position of gamma ray interactions. The detector has at least one module, and each module has a converter for converting gamma rays into charged particles. A scintillator material is provided in the detector for emitting light in response to interactions with the charged particles produced by the converter. A photodetector determines when light has been emitted from the scintillator. A two-coordinate position detector is provided for determining the X, Y and Z coordinates of interaction that produced the detected light.
A controller and signal device are associated with the detector for signaling the detection of emitted light within a photodetector and for activating the position detector. The system disclosed in Bryman I addressed some of the deficiencies of the conventional detectors and provided a gamma ray detector that can be constructed less expensively, requires fewer monitoring instruments for acquiring the required positional data, and which can more accurately determine the X, Y and Z coordinates of the gamma ray interaction.
The conversion of gamma rays in material (including heavy liquids like xenon (Xe), krypton (Kr) and other noble gases) and the production of scintillation light and charged products (electrons and positrons) within such materials are well known to those skilled in the art. Further, software tools are readily available to those working in the art for simulating the interactions of gamma rays and charged particles with the detector matter. Position sensitive detectors for charged particles, such as noble liquid ionization chambers, time-projection-chambers (TPC), and light detection arrays are commonly used instruments having position and energy resolution capabilities that can be similar to those obtained by the apparatus disclosed in more detail below.
Liquid Xe position sensitive ionization detectors with grids such as described by K. Masuda et al., A Liquid Xenon Position Sensitive Gamma-Ray Detector for Positron Annihilation Experiments, Nucl. Instr. Meth. 188 (1981) 629-38; and K. Masuda, et al., Test of a Dual-Type Gridded Ionization Chamber Using Liquid Zenon, Nucl Instr. Meth. 174 (1980) 439-46, each of which is hereby incorporated by reference in its entirety, may be configured to provide sub-millimeter position resolution for low energy gamma rays. Gated time projection ionization chambers (a gas drift device) have been reported. The Columbia University, for example, has disclosed a liquid Xe TPC (E. Aprile, et al., The Liquid Xenon Gamma-Ray Imaging Telescope (LXeGRIT) for Medium Energy Astrophysics, Proceedings-SPIE The International Society For Optical Engineering, SPIE Vol. 2806, pp. 337-48, which is hereby incorporated by reference.
It has also been reported that one of the disclosed embodiments of a liquid Xe ionization TPC achieved a 1 mm position resolution and energy resolution of 5.9% for gamma rays exhibiting 1 MeV energy. Additionally, Lopes et al. have reportedly constructed a liquid Xe ionization detector capable of a transaxial positioning resolution of 1 mm, a depth of interaction resolution of 5 mm, a coincidence time resolution of 1.3 ns, energy resolution at 511 keV of 17% and efficiency of 60% (see, M. Lopes, et al., Positron Emission Tomography Instrumentation: Development of a Detector Based on Liquid Xenon, Proc. Calorimetry in High Energy Physics, pages 675-80 (1999)), which is hereby incorporated by reference in its entirety.
These and other articles present various configurations of instrumentation for collecting ionization signals using pads and wires, gating grids and scintillator triggers that are applied to the problem of measuring charged particle trajectories. These instruments tend to use scintillation light primarily as a fast indicator that a suitable interaction event has occurred, but do not tend to specifically localize the point of interaction.
In the KAMIOKANDE (as described in K. S. Hirata et al., Experimental Study of the Atmospheric Neutrino Flux, PHYSICS LETTERS B, Vol. 205, number 2,3, p. 416-20 (1988)) and other detectors, arrays of photodetectors provided at the surface of light-emitting liquids and/or solids have been used to localize the position of interactions of gamma rays and charged particles. In L. Barkov et al., Search for μ+→e+γ down to 10−14 branching ratio, Paul Scherer Institute proposal. R-99-05.1 (1999), which is hereby incorporated by reference in its entirety lepton-flavor-violating decay μ+→e+γ was studied using a liquid Xe scintillation detector having an array of photo-multiplier tubes surrounding a small volume was demonstrated to give 8 mm full width half maximum (fwhm) position resolution for 1 MeV gamma rays.